I oddly like that theory.Starcraft is the only franchise that acknowledges the existence of Earth as in Brood War the human forces that invaded the sector were the United Earth Directorate.
What if... Stay with me here..
Overwatch takes place in the Starcraft Universe but at a much earlier date and somehow this war of sorts between these heroes encourages Earth to round up all the baddies and send them off into deep space where they colonize and eventually form the terrans.